1. Field of the Invention
The invention relates to a secondary battery temperature-estimating apparatus for estimating a temperature of a secondary battery, and particularly to a technology for accurately estimating the temperature of the secondary battery without using a temperature sensor.
2. Description of the Related Art
In general, in secondary batteries, typified by lithium-ion batteries and nickel-hydrogen batteries, charging and discharging characteristics are degraded as the temperature decreases. When the temperature of the battery is low, it is required to quickly increase the temperature of the battery.
Japanese Patent Application Publication No. H11-329516 (JP-A-11-329516) describes an apparatus for increasing the temperature of a battery. In the temperature increasing apparatus, a resonance circuit is formed by connecting a series circuit consisting of an inductor, a capacitor, and an alternating-current power supply across the battery. The temperature of the battery is increased by making the alternating-current power supply generate an alternating current voltage at the resonance frequency of the resonance circuit.
In the temperature increasing apparatus, almost all the electric power is consumed in the internal resistance during resonance and the temperature of the battery is increased by self-heat generation. It is insisted that this temperature increasing apparatus is capable of effectively increasing the temperature of the battery with minimum electric power consumption (see JP-A-11-329516).
In order to increase the temperature of the secondary battery to a target temperature safely and accurately, it is important to accurately determine the temperature of the secondary battery. The temperature sensor however measures the temperature of the surface of the battery and therefore, the actual temperature of the inside of the battery is not accurately reflected on the result of the detection. In addition, using a temperature sensor leads to an increase of costs. JP-A-11-329516 merely discloses the technology for increasing the temperature of the battery and the above publication includes no teaching concerning the method of accurately estimating the temperature of the battery while the temperature of the battery is increased.